1. Field of the Invention
The present invention relates to a network termination device, and more particularly to a network termination device which determines whether the subscriber loop is in an idle state or busy state by detecting the polarity of a dc voltage supplied from local office equipment.
2. Description of the Related Art
The recent proliferation of the Internet has brought about increasingly widespread deployment of the Integrated Services Digital Network (ISDN) for home use. While conventional subscriber loops are provided originally for analog signal transmission, the ISDN makes it possible to utilize the existing metallic wires to transport high-speed digital signals.
The ISDN basic rate services are based on a reference model shown in FIG. 18, which is defined in the TTC standard JT-G961 (Digital Transmission System on Metallic Local Lines for ISDN Basic Rate Access). TTC stands for the Telecommunication Technology Committee (Japan), and JT-G961 is based on the ITU-T Recommendation G.961. Referring to FIG. 18, a public switched telephone network (PSTN) 1 is a telecommunications network that is accessible to the general public. An exchange termination device 2 and line termination devices 3a to 3c are deployed at a central office. The exchange termination device 2 permits a call from a subscriber to reach the desired destination by making an appropriate circuit connection. Being disposed at the exchange end of each metallic subscriber loop 4, the line termination devices 3a to 3c serve as adapters that perform various operations for digital data communication services. The subscriber loop 4 is physically a twisted pair of copper wires, originally prepared for analog voice signal transmission.
In the system of FIG. 18, the subscriber-side equipment includes: a network termination device 5, subscriber terminals 7a and 7b, a terminal adapter 8, and an analog telephone set 9. Located at the subscriber end of the metallic subscriber loop 4, the network termination device 5 provides various functions as the peer system of the line termination device 3a. The subscriber terminals 7a and 7b, called terminal equipment (TE) in ISDN terminology, are digital telecommunications equipment such as digital telephone sets and G4 standard-compatible fax machines. The terminal adapter 8 serves as a bridge to make the conventional analog telephone set 9 compatible with the ISDN protocol of S-bus 6, converting conventional analog telephone signals to/from ISDN digital signals.
The metallic subscriber loop 4, known as the U interface in ISDN terminology, carries basic-rate digital information between the network termination device 5 and the line termination device 3a. Since the U interface is not internationally standardized, various implementations are possible for the metallic subscriber loop 4. In the United States, echo canceling techniques are used to provide simultaneous full-duplex transmission. Many other countries, on the other hand, use time-compression multiplexing techniques, which are also known as the ping-pong method.
FIG. 19 is a block diagram which presents the details of the line termination device 3 and network termination device 5 in the system of FIG. 18. Here, the symbols "L1" and "L2" represent two individual wires constituting the metallic subscriber loop 4. FIG. 19 shows that the line termination device 3 comprises: a power supply 30, a current detector 31, a DC-AC splitter/combiner 32, a polarity reversing switch 33, a circuit termination unit 34, and a switch controller 35.
The power supply 30 provides the network termination device 5 with electric power. FIG. 20 shows a voltage-current plot representing the output characteristics of the power supply 30. Acting as a load of this power supply 30, the network termination device 5 exhibits a high circuit impedance when in the on-hook (or idle) state, and a lower impedance when in the off-hook (or busy) state. In the former situation, the power supply 30 functions as a constant voltage source, while, in the latter situation, it serves as a constant current source, as can be seen from FIG. 20. The current detector 31 monitors the load current of the power supply 30 and provides the switch controller 35 with a detection signal that indicates whether the current exceeds a predetermined threshold level.
Referring back to FIG. 19, the DC-AC splitter/combiner 32 allows a direct current from the power supply 30 to pass through to the polarity reversing switch 33. Simultaneously, it permits the circuit termination unit 34 to send an outgoing data signal to the polarity reversing switch 33, while preventing the signal from leaking to the power supply 30. The DC-AC splitter/combiner 32 also receives an incoming data signal from the network termination device 5 via the polarity reversing switch 33, and delivers it solely to the circuit termination unit 34.
The polarity reversing switch 33, composed of four switches S1 to S4, manipulates the polarity of the supply voltage, when sending it out to the network termination device 5. More specifically, the polarity reversing switch 33 changes over the supply voltage from normal polarity to reverse polarity, or vise versa, by alternating its internal connection paths according to control commands from the switch controller 35. When the switches S1 and S4 are turned on, a straight connection path is made to place a positive voltage on the wire L1 with respect to the other wire L2. This is referred to as the "normal polarity." Oppositely, when the switches S2 and S3 are turned on, a crossed connection path is established so that a positive voltage will appear on the wire L2 with respect to the other wire L1. This is referred to as the "reverse polarity."
The circuit termination unit 34 performs, for example, bitrate conversion of data signals exchanged between the exchange termination device 2 and network termination device 5. The switch controller 35 governs the polarity reversing switch 33 according to the aforementioned detection signal received from the current detector 31. The switch controller 35 has a dead band which eliminates any possible instability in the current detection during a transitional period when the supply voltage changes from normal polarity to reverse polarity. It also contributes toward increasing noise immunity. Actually, this dead band is realized as a masking function with a predetermined time constant .tau..
FIG. 19 also shows that the network termination device 5 comprises: a DC-AC splitter/combiner 50, a diode 51, a switch 52, a diode 53, an internal power supply 54, a circuit termination unit 55, and a call request detector 56. The structure and function of the DC-AC splitter/combiner 50 are similar to those of the aforementioned DC-AC splitter/combiner 32 in the line termination device 3. In short, only a DC voltage appears at the left-hand port of the DC-AC splitter/combiner 50, while data signals at the bottom port. The diode 51 applies the DC voltage to the switch 52, only when it is with the normal polarity. The switch 52 comprises a semiconductor switch, which is activated by the call request detector 56 when it has detected a call originated from a subscriber terminal (not shown in FIG. 19).
The diode 53 prevents the internal power supply 54 from receiving a DC voltage from the line termination device 3 that is working in normal polarity mode. The internal power supply 54 is typically a DC-DC converter. Operating only with a reverse-polarity DC voltage, it provides a predetermined voltage(s) to other portions of the network termination device 5. As can be seen from the above, the internal power supply 54 will appear to the line termination device 3 as a high-impedance load when a normal-polarity voltage is applied, but as a low-impedance load when a reverse-polarity voltage is applied.
The circuit termination unit 55 performs bitrate conversion of data signals that the line termination device 3 sends and receives to/from terminal equipment including the subscriber terminals 7a and 7b. This circuit termination unit 55 has a power-on reset function that initializes itself when the internal power supply 54 begins to operate. The call request detector 56 detects a call request signal originating at the subscriber terminals 7a and 7b or terminal adapter 8, and controls the switch 52 accordingly.
The next section will explain how the above conventional system operates when a call is originated or received.
(1) When a call is originated
When the network termination device 5 is in an idle state, the line termination device 3 supplies a DC voltage with normal polarity since the switches S1 and S4 in the polarity reversing switch 33 are closed. Hereafter, the terms "normal supply mode" and "reverse supply mode" will be used, if appropriate, to represent two opposite states of the supply voltage: normal polarity and reverse polarity.
FIG. 21 shows how the conventional system handles a call request from subscriber terminals. Standing by in normal supply mode, the network termination device 5 initially appears to the line termination device 3 as a high-impedance load, because its internal switch 52 is open. The power supply 30 thus operates as a constant voltage source, and accordingly, little current flow is observed between the line termination device 3 and network termination device 5.
Suppose here that the subscriber terminal 7a or 7b has placed a call in the above situation, as shown in (A) of FIG. 21. The call request detector 56 then detects this call request signal and turns on the switch 52, resulting in a low-impedance state of the network termination device 5. This triggers an increase of the load current (or subscriber loop current) flowing from the line termination device 3 to the network termination device 5. The current soon exceeds the predetermined threshold level indicated by the broken lines in FIG. 21, thus causing the power supply 30 to switch its operation to constant-current mode. As a result, the increase of the subscriber loop current stops when it reaches a little above the threshold level.
Observing this change in the current value, the current detector 31 notifies the switch controller 35 of the change. The switch controller 35 then activates its integral masking mechanism to filter out spurious transient events. More specifically, it generates a command to the polarity reversing switch 33 to reverse the connection path, only when the observed current value continuously exceeds the threshold for more than the predetermined time period .tau.. The resultant reverse voltage is applied to the network termination device 5. FIG. 21 shows the detection signal (B) produced by the current detector 31, together with the masked detection signal (C) produced inside the switch controller 35.
In reverse supply mode, the diode 51 in the network termination device 5 is turned off, thus blocking any current flowing into the switch 52. On the other hand, the internal power supply 54 becomes active since the other diode 53 is turned on. With the reverse voltage supplied from the line termination device 3, the internal power supply 54 begins to energize the circuit termination unit 55. The circuit termination unit 55 initializes itself by its built-in power-on reset function at the low-to-high transition of the internal supply voltage (D). A call connection procedure begins when this power-on reset is completed.
(2) When a Call is Received
FIG. 22 shows how the conventional system handles an incoming call request. Suppose that the circuit termination unit 34 has received a call request from the exchange termination device 2. To notify the remote end of the presence of an incoming call request, the circuit termination unit 34 controls the polarity reversing switch 33 through the switch controller 35 so that a reverse voltage be sent to the network termination device 5, as shown in (A) of FIG. 22. This is known as the reverse battery signaling.
Now that the line termination device 3 begins to feed in reverse supply mode, the network termination device 5 consumes a larger current because the internal power supply 54 is activated. This causes the current detector 31 in the line termination device 3 to detect the increased current; it then asserts the current detection signal (B), and hence the masked detection signal (C). The switch controller 35 maintains the reverse supply mode.
In the network termination device 5, the activated internal power supply 54 begins to energize the circuit termination unit 55, triggering its built-in power-on reset function, as shown in (D) of FIG. 22. Accordingly, the circuit termination unit 55 can handle the incoming call request.
The following section will now discuss some problems concerning incorrect connection of subscriber loops. As mentioned earlier, the metallic subscriber loop 4 interconnecting the line termination device 3 and network termination device 5 is originally designed for analog voice signal transmission. In the days of analog telephone systems, the polarity of wire connection was not so important. While crossed connection may sometimes happen during the installation of subscriber loops to home, many analog telephone sets would operate correctly regardless of whether the wires are connected straightly or reversely. FIG. 23 shows such a situation where the network termination device 5 is connected to the metallic subscriber loop 4 in a twisted manner. The operation in this case will now be described below, with reference to FIG. 24.
FIG. 24 is a timing diagram which explains what would happen in the case of reverse connection. When a physical cable connection is made, the line termination device 3 immediately starts to feed electrical power to the network termination device 5 in normal mode. From the viewpoint of the network termination device 5, however, the supplied voltage appears as if the line termination device 3 provided a reverse voltage. Accordingly, the internal power supply 54 is activated, thus drawing a current from the power supply 30 over the metallic subscriber loop 4. Now that a relatively large subscriber loop current is observed, the current detector 31 in the line termination device 3 informs the switch controller 35 of the current consumption, asserting a current detection signal (B) shown in FIG. 24. This produces a delayed, or masked, signal (C). The switch controller 35 regards this signal as a call request from the network termination device 5, thus directing the polarity reversing switch 33 to reverse its internal connection. Here, the line termination device 3 attempts to send a reverse voltage to the network termination device 5.
The supplied voltage, however, appears to be of normal polarity, when viewed from the network termination device 5. The internal power supply 54 is now de-energized; the circuit termination unit 55 is unable to operate. The switch 52, on the other hand, is kept open because no call request has been sensed by the call request detector 56. After all, no power is consumed within the network termination device 5. In other words, the network termination device 5 is in a high-impedance state, when viewed from the line termination device 3. Accordingly, the current detector 31 negates the current detection signal to the switch controller 35, and the polarity reversing switch 33 returns to normal supply mode.
As described above, the line termination device 3 has changed the supply mode from "normal" to "reverse," and then back to "normal." It will repeat the same operations endlessly, switching alternately between the reverse and normal supply modes at specific time intervals, as shown in FIG. 24. In this situation, the line termination device 3 cannot accept any call requests from the relevant subscriber terminals, nor can it handle incoming calls properly when so requested by the exchange termination device 2.
To avoid the above oscillation, it is necessary to connect the wires with correct polarity. Particularly when installing a new network termination device 5 at the end of an existing metallic subscriber loop 4, one should remember to check the polarity of wiring at the wall receptacle of interest, before plugging in the cord of the network termination device 5. While portable line testing tools are available, this polarity check can be a factor to increase the time and labor for the installation. When it is revealed that the existing wiring is incorrect, one has to change the polarity by disassembling and reassembling the receptacle. This requires, however, a certain level of expertise that ordinary users may not always have.